High speed dental handpieces are commonly provided with nozzles for directing air and water in spray form towards the tips of the dental burs. The water/air spray performs the dual functions of cooling the work area and clearing debris from that area. To be effective for those purposes, the spray should be focused on the cutting area. Achieving that objective presents difficulties because burs are available in different lengths and configurations. In general, the shanks of such burs have lengths ranging between short, standard, and long, and the cutting tips of such burs vary widely in length, diameter, and configuration. If a dental handpiece has a wide-angle spray pattern to encompass all of the common bur sizes, only a small proportion of the cooling water will actually be directed towards the tip of any given bur, thereby providing a low level of cooling effectiveness for the amount of water discharged. To avoid flooding areas remote from the bur tip with water which performs no useful cooling function, a narrow-angle spray pattern might be provided; however, in that case, the spray may be too narrow to be useful with burs of different length.
The intermixing of water and air to form the aerosol spray may occur either internally or externally of the handpiece. U.S. Pat. No. 3,952,416 shows an internal mixing system in which a portion of the drive air is diverted and mixed with water in a chamber within the neck of the handpiece, the mixture then being discharged through a passage in the head of that handpiece. Other patents disclosing remote or internal mixing are U.S. Pat. Nos. 3,256,603 and 3,175,293. External mixing is typically provided, at least in one commercial line of handpieces, by concentric water and chip air tubes which have their discharge ends disposed immediately adjacent the heads of such handpieces. Simultaneous discharge of air and water from the pressurized lines causes an aerosol spray to be directed towards the dental bur driven by such a handpiece.
The pressure of the water available to form the spray is typically about 10 psi; hence, it is the air under substantially greater pressure that is responsible for propelling the droplets so that they may perform their cooling and clearing functions more effectively. Greater air pressures of approximately 60 psi are available in those systems which provide chip air lines and, consequently, chip air provides an ideal vehicle for the water particles in the development of an aerosol spray. In those systems which do not provide chip air lines, a small amount of drive air at typical pressures of about 30 psi may be bled off to develop the aerosol spray.
While it is advantageous to design a dental handpiece so that it may utilize chip air, if available, in forming the water/air spray and, if not available, will automatically use drive air for that purpose, such a handpiece has in the past been relatively complex and expensive. One such construction involves the interposition of a manifold chamber along the drive air line within the handle of the handpiece. A bleed port extends between the drive air passage and the manifold chamber and flow therethrough is controlled by a poppet valve which is normally closed by the greater pressure of chip air supplied to the manifold chamber by a separate chip air line. Another line leads from the manifold chamber to the spray nozzle. If chip air is available, then the poppet valve is automatically closed and such air flows through the manifold chamber to the spray nozzle, whereas if no chip air is available the valve automatically opens and allows a limited flow of drive air to bleed through the chamber to the nozzle.
Other patents indicating the state of the art are U.S. Pat. Nos. 2,855,672, 2,369,880, and 3,061,930.